In this episode Sam and James discuss the rules for Reactions with a special focus on the Ready Action in D&D. You can find an explanation of this rule in the Player's Basic Rules D&D PDF on pages 70 and 72 or in the Player’s Handbook on pages 190 and 193.
D&D 5e Basic PDF: http://dnd.wizards.com/articles/features/basicrules
James’ blog: http://www.worldbuilderblog.me
Sam’s blog: http://www.rpgmusings.com